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The clever part of SABRE is its ability to use oxygen from the air and burn it in a normal rocket back end. This is achieved by taking air at the front and chilling it down incredibly fast using very, very powerful refrigeration gear running on a closed loop of liquid helium, which dumps the resulting heat into the cryogenic liquid-hydrogen fuel.

Haul stuff by donkey, after a while you're mainly carrying food for the donkeys. It's somewhat the same with orbital launchers - but at least you get the donkeys back with this one.

The trouble with this is that air contains water vapour, and in the normal course of events chilling it down like this would soon block up the SABRE with ice. Preventing frost buildup is one of Reaction Engines' main special sauces, and they have demonstrated that they can do it in the lab to the ESA engineers' satisfaction:

As part of the ESA technical evaluation of the SABRE engine, the design and operating principles of the frost control mechanism were explained to ESA. In addition a number of tests were performed at laboratory scale on request of ESA to demonstrate the repeatability of the frost control. ESA can confirm that the frost control mechanism of the SABRE engine, (at laboratory scale), works and is repeatable. In addition ESA expects these positive results to be repeated on the planned tests of the heat exchanger when it is tested on a VIPER jet engine.

These larger-scale ground tests are planned for this summer, as the Reg previously reported.

Assuming that a SABRE nacelle can be successfully built and flight tested aboard the proposed Nacelle Test Vehicle aeroplane - the ESA endorses this plan - Skylon isn't out of the woods yet. It will still be necessary to build the huge main fuselage and wings, which need to be light, strong, able to resist massive heating, and able to hold hundreds of tonnes of explosive cryogenic-liquid fuel.

The ESA structures team think that Reaction Engines have a decent shot at doing this, however:

Structural design work undertaken by REL does not demonstrate any areas of implausibility, given the relatively benign environment of the flight trajectory.

According to the ESA, the cigar-shaped main fuselage of Skylon is "more akin to that of an Airship than a conventional launcher or aircraft". This makes sense as its designers are facing similar problems to those that the long-ago engineers who built the great rigid airships of the 1930s had to tackle.

Like their illustrious predecessors, the Reaction Engines team need to enclose as much volume as possible with as little weight and internal structure as possible. The old-timers were even - mostly - trying to enclose the same stuff, hydrogen, though in their case in gaseous form rather than liquid.

Overall the ESA can't see right off any reason that a Skylon-style aeroshell, wings etc can't be built.

So we can expect excellent reusable spaceplanes in the reasonably near future, then?

Perhaps. The Skylon is intended to be a commercial design - the reality is, as has long been acknowledged by Reaction Engines, that no European government or assortment of governments is going to stump up the $12bn it will take to get ships into operation.